This invention is generally directed to processes for the preparation of metals, and more specifically the present invention is directed to an improved process for preparing high purity selenium, sulfur, tellurium, and arsenic, by subjecting the corresponding esters to an electrochemical reduction in the presence of an organic media. In one embodiment of the present invention, for example, selenium and tellurium in a purity of 99.99 percent are obtained by subjecting the corresponding pure selenium ester, or pure tellurium ester to an electrochemical reduction in the presence of an organic composition. The resulting high purity metals, particularly selenium, tellurium and arsenic, prepared in accordance with the process of the present invention are useful as photoconductive imaging members, in electrostatographic imaging systems.
The art of xerography as presently practiced, involves the formation of an electrostatic latent image on a photoconductive imaging member which can be in the form of a plate, a drum, or a flexible belt, for example. Materials commonly selected for the photoconductive member contain amorphous selenium, amorphous selenium alloys, halogen doped amorphous selenium compositions, halogen doped amorphous selenium alloys, and the like. These photoconductive members must generally be of high purity, that is, a purity of 99.99 percent or greater, since the presence of contaminants has a tendency to adversely affect the imaging properties of the members, including the electrical properties thereof, causing copy quality to be relatively poor as compared to devices wherein high purity substances are selected.
Numerous complex processes are known for obtaining photoconductive substances, such as selenium, or alloys of selenium, these processes generally being classified as chemical processes, and physical processes. These prior art processes, including the chemical process for obtaining high purity metals involve a number of process steps and undesirably high temperature distillations. Additionally, in many of these processes recycling of the reactants is not achieved. Also, in many instances the prior art processes for recovering selenium, selenium alloys, or other metallic elements from contaminated source materials is complex, economically unattractive, causes environmental contamination in that, for example, various vaporous oxides are formed, and must be eliminated. Furthermore, many of these processes result, for example, in the recovery of selenium or selenium alloys which nevertheless contain impurities that can over an extended period of time adversely affect their photoconductivity. Moreover, flexible photoreceptor devices containing photoconductive compositions prepared in accordance with these processes have a tendency to deteriorate over a period of time and, thus, the selenium or seleniumm alloy used, for example, must be recovered and recycled. Various methods are available for recovering the selenium from the substrate on which it is deposited including heat stripping, water quenching, ultrasonics, and bead blasting.
There is disclosed in U.S. Pat. Nos. 4,007,255 and 4,009,249 the preparation of stable red amorphous selenium containing thallium, and the preparation of red amorphous selenium. In the '255 patent there is disclosed a process for producing an amorphous red selenium material containing thallium, which comprises precipitating selenous acid containing from about 10 parts per million to about 10,000 parts per million of thallium dioxide, with hydrazine, from a solution thereof in methanol or ethanol, containing not more than about 50 percent by weight of water, at a temperature of between about -20 degrees centrigrade, and the freezing point of the solution, and maintaining the resulting precipitate at a temperature of from about -13 degrees Centigrade to about -3 degrees centigrade until the solution turns to a red color. The '249 patent contains a similar disclosure with the exception that thallium is not contained in the material being treated.
In addition to the above described methods for preparing selenium, there are known a number of other processes for obtaining selenium and selenium alloys. Thus, for example, there is disclosed in U.S. Pat. No. 4,121,981 an electrochemical method for obtaining a photoreceptor comprised of a selenium tellurium layer. More specifically there is described in this patent the formation of a photogenerating layer by electrochemically codepositing selenium and tellurium onto a substrate from a solution of their ions in such a manner than the relative amounts of selenium and tellurium which are deposited are controlled by their relative concentrations in the electrolyte, and by the choice of electrochemical conditions. Moreover, there is disclosed in U.S. Pat. No. 4,192,721 the preparation of metal chalcogenides by depositing these materials on a cathode as a smooth film by an electroplating process accomplished at low current densities wherein there is selected a metal salt electrolyte dissolved in an organic polar solvent, and in which is also dissolved the chalcogen in elemental form, with the electrolytic bath being maintained at elevated temperatures.
Further, there is disclosed in U.S. Pat. No. 2,649,409, the electrodeposition of selenium on conducting surfaces. According to the disclosure of this patent selenium may be electrodeposited in its grey metallic form by utilizing an electrodeposition bath containing a supply of quadrivalent selenium cations, that is, cations containing selenium in the quadrivalent state such Se.sup.+4, SeO.sup.+2. Similarly, there is disclosed in U.S. Pat. No. 2,649,410 the manufacturing of selenium rectifiers, selenium photocells, and similar devices wherein grey crystalline metallic selenium is electrodeposited on a cathode from an acidic aqueous solutions of selenium dioxide. More specifically, in the process described in this patent elemental particles of selenium are added to an aqueous acidic solution containing selenium dioxide, the selenium particles being added in a quantity greater than the normal metallic selenium content of the solution, followed by accomplishing an electrodeposition of the resulting treated solution.
Recently, there has been developed processes for preparing selenium and tellurium in high purity wherein the corresponding isolated substantially pure esters are subjected to a reduction reaction with hydrazine or sulfur dioxide, resulting in a product having a purity of 99.999 percent. The details of these processes are described in copending applications, U.S. Ser. No. 404,259, and U.S. Pat. No. 404,257, the disclosure of each being totally incorporated herein by reference.
While the process as described in the copending applications are suitable for the purposes intended, there continues to be a need for other processes for preparing metals such as selenium of high purity. Furthermore, there continues to be a need for improved processes for preparing selenium, tellurium, and arsenic of high purity, 99.99 percent or greater, wherein the electrical properties of the resulting product can be controlled. Additionally, there continues to be a need for processes for obtaining selenium and tellurium in high purity, wherein the reduction of the corresponding pure esters is not accomplished by chemical means, and where there can be obtained products with extended hole transporting properties, and extended electron transporting properties. Moreover, there continues to be a need for the preparation of metals in high purity by subjecting the corresponding pure esters to an electrochemical reduction reaction. Also, there continues to be a need for the preparation of photoconductive materials of high purity by subjecting the corresponding substantially pure metallic esters to an electrochemical reduction in a non-aqueous media.